1. Field of the Invention
The present invention relates generally to optical fiber, and particularly to graded index multimode fibers.
2. Technical Background
The bandwidth of multimode fibers (MMF) is limited mostly by inter-modal chromatic dispersion. To minimize the inter-modal chromatic dispersion, MMF are designed with graded index α-profiles. For optimum bandwidth, the parameter α is about 2. However, a practical MMF has a cladding that truncates the ideal alpha profile. For higher order modes near the cutoff frequency whose effective refractive index is close to that of the cladding, the time delay deviates from that of the infinite alpha profile. In the past, these modes were not of particular concern because they had high loss as a result of perturbations such as bending and cladding absorption.
However, as fiber glass and coating technologies have improved, losses due to micro-bending and cladding absorption have reduced significantly. As a result, some of the higher order modes can survive over the length of practical applications, which limits the multimode fiber bandwidth.